Similarly to uvm_fault_upper_lookup(), uvm_fault_lower_lookup() enters
existing neighbor pages for lower layers faults (with an UVM object).
This is where objects shared among many processes (binaries, libraries,
etc) are contended.

To reduce the cost of entering neighbor pages ahead, use the same trick
already present in uvm_fault_upper_lookup().  Call pmap_extract(9) to
know if a page is already present in the memory space before entering it.

The 'goto next' is questionable.  I did it this way because it will
disappear in the next step.  My current goal is to be able to call this
function in parallel for a given UVM object.  That implies not setting the
PG_BUSY flag on pages returned by PGO_LOCKED.  So the goto will soon be
a continue.

While here add a cheap `wire_count' check before grabbing the heavily
contended pageq mutex. 

Finally use the same `entered' technique to decide if pmap_update()
should be called or not.

ok?

Index: uvm/uvm_fault.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_fault.c,v
diff -u -p -r1.146 uvm_fault.c
--- uvm/uvm_fault.c	27 Nov 2024 10:58:07 -0000	1.146
+++ uvm/uvm_fault.c	28 Nov 2024 09:02:36 -0000
@@ -1115,8 +1115,9 @@ uvm_fault_lower_lookup(
 {
 	struct uvm_object *uobj = ufi->entry->object.uvm_obj;
 	struct vm_page *uobjpage = NULL;
-	int lcv, gotpages;
+	int lcv, gotpages, entered;
 	vaddr_t currva;
+	paddr_t pa;
 
 	rw_enter(uobj->vmobjlock, RW_WRITE);
 
@@ -1135,6 +1136,7 @@ uvm_fault_lower_lookup(
 		return NULL;
 	}
 
+	entered = 0;
 	currva = flt->startva;
 	for (lcv = 0; lcv < flt->npages; lcv++, currva += PAGE_SIZE) {
 		if (pages[lcv] == NULL ||
@@ -1155,18 +1157,14 @@ uvm_fault_lower_lookup(
 			continue;
 		}
 
-		/*
-		 * note: calling pgo_get with locked data
-		 * structures returns us pages which are
-		 * neither busy nor released, so we don't
-		 * need to check for this.   we can just
-		 * directly enter the page (after moving it
-		 * to the head of the active queue [useful?]).
-		 */
+		if (pmap_extract(ufi->orig_map->pmap, currva, &pa))
+			goto next;
 
-		uvm_lock_pageq();
-		uvm_pageactivate(pages[lcv]);	/* reactivate */
-		uvm_unlock_pageq();
+		if (pages[lcv]->wire_count == 0) {
+			uvm_lock_pageq();
+			uvm_pageactivate(pages[lcv]);
+			uvm_unlock_pageq();
+		}
 		counters_inc(uvmexp_counters, flt_nomap);
 
 		/* No fault-ahead when wired. */
@@ -1181,16 +1179,19 @@ uvm_fault_lower_lookup(
 		(void) pmap_enter(ufi->orig_map->pmap, currva,
 		    VM_PAGE_TO_PHYS(pages[lcv]) | flt->pa_flags,
 		    flt->enter_prot & MASK(ufi->entry), PMAP_CANFAIL);
+		entered++;
 
 		/*
 		 * NOTE: page can't be PG_WANTED because
 		 * we've held the lock the whole time
 		 * we've had the handle.
 		 */
+next:
 		atomic_clearbits_int(&pages[lcv]->pg_flags, PG_BUSY);
 		UVM_PAGE_OWN(pages[lcv], NULL);
 	}
-	pmap_update(ufi->orig_map->pmap);
+	if (entered > 0)
+		pmap_update(ufi->orig_map->pmap);
 
 	return uobjpage;
 }